1. Field of the Invention
The present invention generally relates to a display system and, more particularly, to an image display apparatus such as a flat-type display utilizing electron emission by polarization reversal of a ferroelectric material.
2. Description of the Related Art
In recent years, due to the great needs to notebook type personal computers, portable game machines, and the like, the production and sales of image display elements are increased. More specifically, liquid crystal display elements are more popularly used than other solid image display elements because of the low power consumption of the liquid crystal display elements.
The liquid crystal displays are roughly classified into simple matrix liquid crystal displays and active matrix liquid crystal displays. Although the simple matrix liquid crystal displays are advantageously used for high-density integration because of their simple structures, the simple matrix liquid crystal display elements have crosstalk to a non-selected cell, and an increase in resolution which is an object of the high-density integration cannot be achieved. In contrast to this, in the active matrix liquid crystal displays, crosstalk to a non-selected cell can be suppressed without posing any problem, and an image having a high resolution can be obtained, thereby considerably improving image quality. In this manner, a large number of active matrix liquid crystal displays have been used in recent years.
However, in these liquid crystal displays, the following problems are posed. First, the liquid crystal displays are not self-emission type displays. For this reason, although the liquid crystal display elements are improved using back light electric luminescence (EL) or a back light fluorescent tube, the service life and power consumption of the back light electric luminescence and back light fluorescent tube pose a problem. In addition, a liquid crystal display has a field angle narrower than that of each of other display devices, i.e., about 30.degree., and has poor time response.
In addition, especially, an active matrix liquid crystal display is manufactured in complex manufacturing steps, and the production cost of the active matrix liquid crystal display is high.
The liquid crystal displays having the above drawbacks are not satisfactorily used in image display apparatuses which are popularly used in the field of information industries, and image displays free from the above drawbacks are required. Although an image display apparatus using a cathode-ray tube is excellent in a field angle, time response, and a resolution, the image display apparatus is a vacuum tube apparatus and has poor portability and high power consumption.
As described above, although a conventional image display element has been improved, there is no image display apparatus which can simultaneously satisfy a high image resolution, excellent time response, a wide field angle, a self-emission property, low power consumption, and low cost. The image display apparatus which satisfies the above conditions is demanded.
On the other hand, an EL element is developed, as a self-emission type of light-emitting device which satisfies the low power consumption.
In this EL element, a thin film is inserted between an phosphor (light-emitting) film and a thick insulating film on the phosphor (light-emitting) film side, and an intermediate electrode is inserted between the light-emitting film and the thick insulating film on the thick insulating film side. Note that a transparent electrode consisting of a metal such as Al or Au or ITO (Indium Tin Oxide) may be used as the intermediate electrode, or the intermediate electrode may consist of an n-type semiconductor in which a donor is very heavily doped. However, when the metal or semiconductor is used, the intermediate electrode from which light is extracted must have a small thickness enough to transmit the light.
In this structure, when a voltage is applied across a back electrode and the transparent electrode, and an electric field is applied to the phosphor (light-emitting) film, electrons from the intermediate electrode tunnel through the thin insulating film to be injected in the phosphor (light-emitting) film. The injected electrons are accelerated by the electric field generated in the phosphor (light-emitting) film and collide with a luminescent center in the phosphor (light-emitting) film so as to excite the luminescent center.
When the intermediate electrode is not formed, electrons injected in the phosphor (light-emitting) film are supplied from a level (trap) located at the interface between the insulating film and the phosphor (light-emitting) film. Therefore, in the same electric field, the number of injected electrons is larger when the intermediate electrode is formed than when the intermediate electrode is not formed, and light emission luminance is increased.
In the general EL element as discribed above, carriers are supplied by forming a trap site or a space charge region in a high electric field. However, in order to obtain carriers in only this electric field, an electric field of several MV/cm must be applied to extract the carriers by a tunnel phenomenon, or hot carriers must be generated. In this manner, it is conventionally very difficult to extract carriers by the tunnel phenomenon or by the generation of hot carriers. In addition, since a drive voltage is very high, i.e., about 100 V, the EL element is not practically used in place of a liquid crystal display element.